Screw compressor



0:5 20, 1970 E. KODRA 3,535,051

SCREW COMPRES SOR Filed Sept. 6, 1968 5 Sheets-Sheet 1 r O N S Oct. 20, 1970 E. KODRA 3,535,057

SCREW COMPRESSOR Filed Sept. ,6, 1968 5 Sheets-Sheet 2 Inventor Esper Kodra.

Oct. 20, 1970 KODRA SCREW COMPRESSOR 5 Sheets-Sheet 5 Filed Sept. 6, 1968 I O t n e v M Esper Kodra.

0d. 20, 1970 KQDRA SCREW COMPRESSOR Y 5 Sheets-Sheet 4 Filed Sept. 6, 1968 r 0 M e v M Esper Kodra.

Oct. 20, 1970 E. KODRA 3,535,057

SCREW COMPRESSOR Filed Sept. 6, 1968 5 Sheets-Sheet 5 as 10 87 I 92 94 5 i Separator 4-,-

a 3 c? 04L flooded u g Wcmerflooded, 'U :i I (n Rotor Tip Speed (Male) Fig .5.

Inventor Esper Kodra.

United States Patent 0.

3,535,057 SCREW COMPRESSOR Esper Kodra, 118 Belmont Court, Michigan City, Ind. 46560 Filed Sept. 6, 1968, Ser. No. 758,032 Int. Cl. F04c 17/12, 29/04 US. Cl. 418-1 9 Claims ABSTRACT OF THE DISCLOSURE A screw compressor provided with an abradable and/or deformable coating, on one or both rotors to provide for running into a desired close fit for operation as a water flooded compressor and having housings for the screw supporting bearings separate from the screw enclosing housing to prevent cross leakage of injected water and bearing lubricant.

The screw compressor of this invention is of a type well known in the art wherein one or more male helical rotors mates with one or more female helical rotors to provide variable volume compression pockets progressing axially from one end of said rotors to the other to provide positive displacement compression for gaseous media such as air. This compressor is similar to that disclosed, described and shown in US. Pat. 3,245,612 (Nillson) except for hereinafter delineated significant differences.

Screw compressors of the general type of this invention have been used for many years and have served the purpose for which they were designed although presenting the following problems at least some of which are solved by the structure according to the present invention:

Dry screw compressors:

(1) Extremely close machining tolerances necessary to reduce leakage along the sealing line;

(2) Extremely high speed operation to reduce leakage along the sealing line;

(3) The mandatory use of timing gears to prevent one rotor from touching another;

(4) High temperature produced due to high speed operation and lack of coolant in the compression chamber will in most cases require two stages with an intercooler for pressures greater than sixty (60) pounds per square inch;

In oil flooded compressors:

(5) Power loss due to churning of oil in the compression chamber;

(6) The presence of oil in the compressed gas;

(7) Relatively high delivery temperature due to the necessity of keeping the discharge temperature above the moisture condensation temperature.

The screw compressor of this invention being provided with rotors coated with an abradable or deformable coating widens the allowable machining tolerances at least in the direction of interferring rotors since the rotors necessarily very closely fitting can, without harm, be in slight interference i.e. have negative clearance therebetween when first installed in the housing and upon being slowly rotated the coating is abraded or deformed to provide the necessary operating clearance after a relatively small number of slow rotations.

Separate housings being provided for the bearings eliminates the possibility of lubricant from the bearings leaking into the compression chamber to contaminate the gaseous compressed product. Such separate housings also make it possible to employ replaceable compression cavity seals wherein the wearing complements are replaceable without removing the rotors from the bearings.

Operation of this compressor as a water flooded machine allows high mass flow to be used which coupled 3,535,057 Patented Oct. 20, 1970 ice with the high specific heat of the water will give nearly isothermal compression with the attendant advantages of low specific horse power and longer life for elements of the compressor.

Water flooded operation also assures completely oil free an.

The low viscosity of the water as compared to oil gives lower specific power requirements and allows higher rotational speed than with oil flooded machines.

These and other advantages of the screw compressor of this invention will become more readily apparent upon consideration of the following description and drawings in which:

FIG. 1 is asectional view taken substantially on the common plane of the rotor center lines of a two rotor screw compressor constructed according to the principles of this invention;

FIG. 2 is a side elevational view of the compressor of FIG. 1;

FIG. 3 is a fragmentary sectional view taken substantially on line 3--3 of FIG. 1;

FIG. 4 is a fragmentary end view taken substantially on line 44 of FIG. 3;

FIG. 5 is a left end elevational view of the compressor of FIG. 1;

FIG. 6 is a sectional view taken substantially on line 6-6 of FIG. 1;

FIG. 7 is a schematic representation of a method of employing the compressor of this invention; and

FIG. 8 is a graphic representation of the specific power vs. rotor tip speed relationships of screw compressors.

Referring now to the drawings a preferred embodiment of the compressor of this invention generally indicated at 10 is shown in FIG. 1 as comprising a housing assembly generally indicated at 12 having a chamber 15 within a compression chamber housing 14 mounted between mated end plate and seal housings 16 and 17, discharge and inlet housings, respectively, to the right and left of chamber housing 14 as viewed in FIG. 1. Spaced outwardly to the right as viewed in FIG. 1 is a separate bearing housing 18 rigidly secured to or preferably made integral with a plurality of axially extended mounting ribs 19 of the discharge seal housing 16 to provide bearing support as hereinafter made plain. The substantially horizontal ribs'such as 19' are provided with openings 21 to drain off accumulated liquids leaking from any of the housings along the shafts 28 and 38.

To the extreme right as viewed in FIG. 1 the right hand end of the housing assembly 12 is completed by a cup shaped timing gear housing 20 secured to the right hand end of the housing assembly 12 as by cap screws or other threaded retaining means.

To the left of the compression chamber housing 14 the inlet seal housing 17 is rigidly connected to or made integral with a separate bearing and drive gear housing 22 by ribs 19 and 19' as above described and a cup shaped end plate and oil pump mounting 24 enclose the left hand end of housing assembly 12. On the outer surface of end plate 24 an oil pump 25 is mounted for purposes to be clear.

Rotatably mounted in the housing assembly 12 is a pair of screw compressor rotors of a type well known in the art being a male rotor 26 and a female rotor 36 upper and lower as viewed in FIG. 1, respectively. The male rotor 26 comprises a helix portion 27 mounted on or made integral with a shaft 28 extending through the seal housings 16 and 17, respectively, to bearing portion 29 and 30 to the right and left, respectively, of the sealing portions of shaft 28 and extended therebeyond to gear mounting end portions 31 and 35 at the right and left hand ends of the shaft 28, respectively.

Rotatably mounted within and extending outwardly from the end plate 24 is a drive shaft 32 (see FIG. 2) tapered and provided with a keyway or other means of connection to an internal combustion engine or electric motor or other power means for rotating the shaft 32. Mounted on the portion of the shaft 32 extending within the drive gear housing 22 is a suitable drive gear 33, shown only as a pitch line circle in FIG. 5, as being in toothed engagement with a suitable driven gear 34 seen in FIG. 1 as being mounted on shaft 28 in driving engagement with the gear mounting portion 35 of the male rotor shaft 28 so that power applied to the outboard end of the drive shaft 32 is transferred through the drive gear 33 and the driven gear 34 to the helical portion 27 of the shaft 28 to provide powered rotation thereto as required.

The female rotor 36 similarily comprises a helix portion 37 mounted on a rotor shaft 38 extending through the discharge and inlet sealing housing 16 and 17 respectively and including right and left end bearing portions 39 and 40, respectively, rotatably supported in the bearing housings 18 and 22 with a gear mounting portion 41 on the right hand extreme end thereof. A suitable timing gear 43 is mounted on portion 41 in driving relationship therewith and in toothed engagement with a smaller timing gear 45 mounted on the gear mounting portion 31 of the male rotor shaft 28 in keyed relationship therewith. The timing gears 43 and 45 provide for timed rotation of the male and female rotors 26 and 36 respectively so that when the rotors are 4-lobed male and 6-lobed female as seen in FIG. 6 the 6-lobed female rotor 36 will rotate at only two-thirds the speed of the 4-lobed male rotor 26 to maintain the timing and clearance of the mating rotors 26 and 36 as hereinafter more fully described.

The left hand end of the male rotor shaft 38 is provided with a driving connection shown as a slot and tongue arrangement 47 for supplying power to the shaft 48 of the oil pump 25 in a manner well known in the art. The oil pump 25 is of course suitably connected to the gear and bearing housings and a suitable oil sump (not shown) to furnish circulating oil to the gears and bearings of the compressor 10 in a well known manner.

Mounted on the rotor shafts 28 and 38, respectively, Within the respective seal housings 16 and 17 is, within each housing, a pair of sealing assemblies generally indicated at 50, shown in enlarged detailed section in FIG. 3 and enlarged end view in FIG. 4. Since the four sealing assemblies 50 are identical and used for identical purposes only that one mounted within the seal housing 16 on the female rotor shaft 38 and shown in section in FIG. 3 will be described. As best seen in FIGS. 3 and 4 the shaft 38 of the female rotor 36 is provided with a shoulder portion 52 adjacent the helix portion 37 thereof, 'which shoulder portion 52 is rotatably received in a diametrically reduced bore portion 54 of a stepped through bore 53 in the seal housing 16 around the right hand sealing portion of the shaft 38. The shoulder portion 52 being of a larger diameter than the remainder of the shaft 38 provides a should 51 against which a spring seat collar 56 slidably received by the shaft 38 is snugly seated and held in position by a somewhat compressed helical compression type spring 58 maintained in a partially compressed condition between the spring seat collar 56 and a second seat collar 60 mounted on the exterior of a flanged hollow cylindrical elastomeric bushing 62 snugly received by the shaft 38 and rotatable therewith. Within the spring seat collar 60 and held in contact with the elastomeric bushing 62 is a hard smooth disc shaped element 64 made of smooth impervious material such as ceramic or hardened steel. It is to be understood that with the snug fit f the elastomeric bushing 62 on the shaft 38 and the biasing of the bushing 62 against the disc element 64 and the contact of the spring seat collar 56 with the shoulder 51 the elements so far enumerated i.e. collar 56, spring 58, bushing 62, collar 60 and disc element 64 will all 4 rotate with shaft 38 whenever the compressor of this invention is in operation.

The stationary elements of the seal assembly 50 comprise a U-shaped metal retaining element 66 having a U- shaped two sided angle slot therein providing a generous clearance around the shaft 38 and mounted on the outward facing surface of the seal housing 16 as by cap screw 67 or other retaining means.

Snugly received within the bore 53 and held in place by retaining element 66 is an externally shouldered, internally tapered retainer ring 68 having on its exterior surface a suitable peripheral groove suitably receiving a sealing element such as an O-ring 69 to provide fiuid tight relationship between the housing 16 and the retainer ring 68. The interior bore of the retainer ring 68 is tapered from left to right as seen in FIG. 3, and has a minimum diameter at its right hand end large enough to provide the same generous clearance around the shaft 38 as that provided by the retaining element 66. Snugly matingly received within the tapered bore portion of the retaining ring 68 is a carbon ring 72 made up of two mated halves through the instrumentality of having been made as a single piece then cut approximately half way through in the axial direction from right to left as seen in FIG. 3. This cut is shown as a kerf 73 in the carbon ring element 72 followed in the same axial direction by a break 74 (see FIG. 1) showing where the original ring element 72 was broken into two halves which are remated in the assembly of the sealing assembly 60. With the two halves held together by a suitable sealing element such as an O- ring 70 seated in a suitable groove on the tapered surface of ring element 72 the O-ring 70 makes contact with the inside taper of the retainer ring 68 to provide a fluid tight junction between the carbon ring 72 and the retainer ring 68. The left hand face of the carbon element 72 is shown as having a raised narrow ring surface portion 76 to provide for high pressure contact with the disc element 64 during an initial running in period to establish the proper sealing effect therebetween.

Examination of the elements of sealing assembly 50 as hereinabove described shows that the compression chamber 15 within the housing 14 can communicate with the interior of the bore 53 but that interior of the bore 53 is sealed off from the atmosphere exterior thereto by the elastomeric bushing 62 held in sealing contact against the shaft 38 and against the disc element 64 which although rotating with the shaft 38 is in sealing contact with the carbon ring 72 along the ring surface 76. The stationary carbon ring 72 is as has been described in sealing contact with the stationary retainer ring 68 in turn in sealing contact with the housing 16 along the interior of the bore 53. It is thus seen that the only frictional contact subject to relative motion and resultant wear is the polished surface of the ceramic or hard steel disc element 64 against the surface 76 of the carbon element 72. At this point wear must inevitably take place and when such wear progresses far enough the disc element 64 or the collar can come into undesirable contact with the ring element 68 with consequent damage to both the disc element 64 and the ring 68. However with a reasonable maintenance schedule of inspection the approach of this condition can readily be ascertained and the following replacement action can be applied. When the carbon ring 72 has become worn to an undesirable extent the cap screws 67 are to be removed allowing the retaining element 66 to be taken off the shaft 38 and out of the way. The ring 68 can then be moved to the right toward the bearing housing 18 far enough so that the worn carbon ring 72 can be moved to the right out of the bore 53 the O-ring removed from around the carbon ring 72 and the worn halves of the carbon ring 72 allowed to fall apart and be discarded. A new carbon ring 72 having been already partially cut through and broken as described for the original carbon ring 72 can be assembled around the shaft 38 and the O- ring 70 placed in the groove around the new carbon ring 72 to hold the halves in proper relationship while the ring element 68 is brought from the right hand position into contact with the outside of the ring element 72 whereupon, the retaining element 66 being placed over the outside of the ring 68 and pushed up against the outer surface of the housing 16, replacement of the screws 67 will complete the replacement of the carbon ring 72 without disturbance to the shaft 38 within the various housing portions and an entirely new wearing surface will thus be provided at a minimum expense of down time and labor.

As seen in FIG. 1 the shafts 28 and 38 are supported at the right hand end wherein suitable thrust and radial hearings in double acting pairs on the portions 29 and 39 of shafts 28 and 38 respectively provide both radial and axial stability to the rotor shaft in a well known manner. It is to be noted that the timing gears 43 and 45 are so mounted on the shafts 28 and 38 as to be movable thereon to adjust the mating of the helixes as necessary. The left hand ends of the shafts 28 and 38 are supported in roller bearings 80 having large radial load capacity particularly for the support of shaft 28 on its drive end wherein heavy radial loads due to the toothed engagement of gears 33 and 34 are imposed on the bearing 80 supporting the shaft 28 at the bearing mounting portion 30.

Due to the separation of the bearing housings 18 and 22 from the chamber housing 14 and with the gaps between the seal housings 16 and 17 and the bearing housings 18 and 22 extending down to a point well below the peripheries of shafts 28 and 38 any leakage of oil from the bearing chambers 18 and 22 or of water from the seal chambers 16 and 17 will be drained through openings 21 to a point of disposal outside of the compressor so that there will be no cross leakage or contamination of oil by water or vice versa. Such effective separation makes it possible to guarantee completely oil free compressed air from the compressor of this invention.

As best seen in FIG. 6 the respective helix portions 27 and 37 of the rotors 26 and 36 are completely enveloped in coatings 82 and 84 respectively of suitable non-metallic material such as Kynar (a registered trademark of Penn Salt Chemicals Corp), Teflon (a registered trademark of Du Pont) or a suitable epoxy formulation. The particular coating used should have the following characteristics: a dependable metal-plastic bond; good resistance to erosion; resistance to corrosion by water; withstand temperatures of 250 F. without softening; low plastic to plastic cohesion; a coeflicient of expansion as near metal coefficient of expansion as possible; and low porosity. Further desirable characteristics are that the material be furnished in reasonable quantities with internal uniformity and uniforrnity from one batch to another; that the material be non-toxic in all conditions and chemically stable. It is furthermore important that the coating material be resilient to a reasonable degree and beyond such degree should be permanently displaceable, i.e. abradable or deformable, so that when two rotors, at least one being coated, are run in they can at first be in interference with a reasonable amount of resilience to allow them to turn relative to each other and with the displaceable characteristic assuring that interference beyond a certain amount will be non-recurring after the first few turns of the rotors so that they can be run in to sufiicient clearance without damaging each other.

It is to be noted with respect to the rotor coating material; it is not necessarily resistant to high temperatures, i.e. above 300 F., nor is it necessary that the coeflicient of friction be particularly low or the tensile strength be particularly high when timing gears are used.

The hardness of the coating should be suflicient to resist deformation by high pressure air or water but soft enough to allow stray metallic particles such as filings to be embedded in the coating rather than being retained on the surface to cause damage to the opposite rotor.

It is further to be noted that one of the rotors may well be metal coated as long as such coating would be of a low friction, corrosion resistant nature with the possibility of being displaceable according to the above description.

With coatings meeting the parameters above set forth it is possible to use rotors rough machined to a dimension somewhat smaller than desired for the finished rotor after which one or more layers of coating material is applied to the rotor to bring the size of the rotors up to line for line dimensions with some possible interference. After being so prepared the rotors are to be installed in the compressor housing with gears and bearings adjusted to provide the correct relationship of one rotor to the other. The rotors will then be slowly rotated to provide a run in period during which the coatings might be in interference from one rotor to the other and from rotor to housing with any necessary permanent deformation of the coating taking place during this period. With proper run in of suitably coated rotors there will be developed a very close fit between the rotors and between rotors and housing with less clearance than would normally be developed to machining to usual tolerances.

The reduced clearances developed by such coating and run in methods will allow slower speed with less leakage resulting in lower operating temperature because of less recompression of hot gases from high pressure volumes of the screw pockets leaking into the lower pressure areas.

It is to be noted that in the preferred embodiment shown in FIGS. 1 and 2 timing gears 43 and 45 are indicated as being desirable, however, such timing gears are not absolutely necessary and may be dispensed with in certain embodiments of this invention wherein the coatings have the requisite characteristics to withstand rotor to rotor contact and the amount of water used is sufficient to provide for the driving of one rotor by the other without timing gears intervening. Thus timing gears 43 and 45 do not form an indispensable part of this invention although self-lubricating properties of the coating material will be required if no timing gears are used.

It is to be noted that in addition to allowing wider machining tolerances the coating also prevents metal to metal contact with attendant scoring and galling resulting in serious damage from metal rotor contact in compressors of the prior art.

Operation of the compressor 10 of this invention hereinabove described as a water flooded screw compressor is made possible by the non-corroding nature and the close fitting of the coated rotors. It is obvious that if the rotors at least in the helix portion were not coated the use of water mixed with air thereon would soon cause damaging corrosion intolerable in such a machine. The rotors can be made of some non-corrosive non-ferrous metals rather than being coated to prevent corrosion. Larger clearances must of course be provided in order to avoid any possibility of metal to metal contact. It is further to be noted that the low viscosity of the water makes it necessary to have closer fitting rotors and casing than would be necessary with the higher viscosity oil with which the liquid flooded compressors of prior art are supplied.

A further characteristic of the compressor of this invention making water flooded operation possible resides in the physical separation of the shaft portions devoted to sealing the high pressure cavity from the shaft portions providing for bearing support of the rotors so that the bearings may be bathed in oil while the compression cavity is partially filled with water without any cross contamination of the two liquids.

The use of water flooding made possible by the above described design features of the structure of this invention gives rise to several methods of compressor operation either impractical or completely impossible with either dry compressors or an oil flooded operation.

The first of these methods of operation is schematically represented in FIG. 7 wherein the compressor 10 of this invention represented by the conventional symbol for a pump is suitably connected to atmosphere by an 7 air intake conduit 86 and in turn communicates through an outlet conduit 87 with a water separator 92 of any suitable type wherein water mixed with air from the compressor 10 can be separated by simple mechanical means. The water separated from the air in separator 92 is returned to the pump 10 through a water conducting conduit 93 communicating between the water catching portion of separator 92 and a heat exchange means 88 suitably supplied with cooling water through inlet and outlet connections 89 and 90 suitably connected to a water supply and disposal means (not shown) and in turn communicating through a conduit 91 with the chamber of the compressor 10. The separator 92 is also suitably connected to a compressed air receiver or other air receiving and utilizing means (not shown). The conduit 91 also communicates through a valve 95 with a suitable source of makeup water to be used to replace any of the water in this system which might be picked up by the air and passed onward through the connection 94 under conditions of low humidity for example. The conduit 91 communicates with the interior of the chamber 15 within the chamber housing 14 in any suitable manner as for example by injection channels such as a plurality of bores 96 (see FIG. 6) similar to those shown in the above cited Nillson patent (3,245,612,

column, lines 10 through 23) which communicate between the outlet side of chamber 15 along the line of intersection of the intersecting bores of which the chamber 15 is formed and an elongated manifold passageway 97 in turn connected to the conduit 93.

In operation of the system shown in FIG. 7 with the compressor 10 operated at rated speed, air at atmospheric pressure and temperature of probably 70 F. is taken into the compressor 10 along with a suitable amount of water to give a mass flow ratio of liquid to gas of a suitable selected value from 10 to 1 up to a maximum of to 1. With such high values of mass flow ratio and the high specific heat value of water the mixture of air and water emerging from the compressor 10 through the outlet connection 87 although compressed to perhaps 100 pounds per square inch pressure will be at a slightly elevated temperature of perhaps 90 P. so that the compression has taken place substantially isothermally with a polytropic process exponent of 1.05 to 1.1 as compared to the accepted figure for adiabatic compression of 1.4 and the theoretical exponent of 1.00 for isothermal compression.

It is further to be realized that, if water having a low enough temperature is economically available, the high mass flow ratio will make it possible to achieve completely isothermal compression with an exponent of 1.0 and even to go beyond the isothermal conditions so that some cooling of the air takes place during compression yielding a polytropic process exponent of as low as 0.9 with concomitant increase in compressor efficiency. For purposes of this invention compressor operation under conditions yielding a polytropic process exponent in the range of 0.9 to 1.1 will be specified as substantially isothermal compression.

Within the separator 92 simple mechanical means such as centrifugal action separates the water from the air which passes on through the connection means 94 into a compressed air receiver or other point of use as well known in the art. As shown in FIG. 7 the conduit 93 conducts the water from the separator 92 back to the compressor 10 to enter into the cycle again. The reason for recirculating the water through the conduit 93 and the pump 10 would reside in a desire to prevent the formation of deposits in the compressor with an original filling of distilled water, condensed water from the air stream will supply injection 'water to the compressor 10. The water supplied through the inlet conduit 89 of the heat exchanger 88 and exhausted therefrom through the outlet conduit 90 has no requirements for particular 8 purity as it never comes into contact with the air stream and will satisfy the conditions of heat exchange if its temperature is F. or less and there is not enough contamination therein to cause plugging up of the heat exchange passages.

It is to be realized that the use of a closed circulation system through the conduit 93 is not necessary to the operation as long as proper arrangements are made to supply the suitable amount of liquid to the compressor 10 and to separate the water from the air in the separator 92.

As is evident from the curves shown in FIG. 8 wherein specific power is plotted against male rotor tip speed the higher rotor tip speed of the water flooded compressor as compared to oil flooded operation plus the substantially isothermal compression gives a very low specific power requirement. The substantially isothermal compression holding the delivery temperature of the compressor at a comparatively low value also contributes to better life of the seals than is possible with the higher delivery temperature found in either dry or oil flooded compressor operation. Separation of the water from the air is a simple mechanical process as compared with the complicated thermal or chemical operations necessary to separate oil and the vapors thereof from compressed air furnished by an oil flooded compressor. This low temperature operation means also that the air leaving separator is at a saturation point corresponding to this temperature. This low moisture content can be obtained from other known compressor types only if air is passed through an after cooler. A further point of superiority of water over oil as a flooding agent resides in the fact that a mixture of water and air can never be explosive or even combustible.

A second method of operation of the compressor 10 of this invention is similar to that above described excepting only that the heat exchanger 88 is eliminated and the conduit 93 communicates with a place of water disposal (not shown). Thus, water added through the valve 95 is taken into the compressor in the usual manner and the air water mixture travels to the separator 92 wherein the water is separated therefrom and the air passes on through the conduction means 94 to the place of use of compressed air as before with the separated water being disposed of in any suitable manner. In this second method of operation the temperature of the water air mixture is controlled mainly by the amount and temperature of water added and is allowed to remain at whatever elevated temperature is developed in the compressor, perhaps F. as it passes through the separator and into the receiver. For certain purposes an added 20 or so of temperature rise is not deleterious and water of suflicient purity to be used in direct contact with the air stream is readily and cheaply available so that the added cost of the heat exchanger and the controls therefor are not warranted by the cost of the flow through water being used to flood the compressor in the non-recirculating method.

A preferred embodiment of the compressor of this invention having been hereinbefore described and herewith illustrated it is to be realized that variations in the specific structure are envisioned and contemplated as further embodiments of the principles of this invention. It is therefore respectfully requested that this invention be interpreted as broadly as possible and limited only by the scope of the appended claims.

What is claimed is:

1. In a method of operating a screw compressor to produce compressed gas by use of a compressor having a pair of interfitting screw rotors comprising the steps of supplying a flow of gas to the inlet portion of said rotors, compressing said gas by action of said rotors, discharging said gas from said compressor and delivering said gas to a point of use for said said gas; the improvement comprising su plying a pressure flow of water to the outlet side of said rotors to provide a mixture of gas and water containing enough water so that said compressing is accomplished without substantial temperature rise.

2. The method of operating a screw compressor as specified in claim 1 including the additional step of separating said water from said gas.

3. The method of operating a screw compressor as specified in claim 2 additionally comprising the step of returning said separated water to said compressor to form at least a portion of said supply for another cycle of operation.

4. The method of operating a screw compressor as specified in claim 3 additionally comprising the step of cooling said separated Water before returning said separated water to said compressor.

5. The method of operating a screw compressor as specified in claim 3 or claim 4 wherein said supplying of gas and said supplying of Water and said compressing of said mixture take place along a path spaced from and sealed off from the means for rotating and supporting said rotors.

6. The method of operating a screw compressor as specified in any of claims 3 or 4 or 1 wherein said compression of the mixture takes place substantially isothermally with a polytropic process exponent in the range from 0.9 to 1.1.

7. In a screw compressor of the interfitting helical rotor type having at least a pair of such rotors rotatably supported in bearings with respective central helical portions located in a central sealed compression chamber portion of an elongated housing member, the improvement comprising: a shaft portion extending outwardly from each end of each rotor helical portion through high pressure and low pressure ends of said sealed chamber, respectively: each end of each shaft being supported in respective bearing means; at least those of said bearing means at the high pressure end of each shaft being axially spaced from said chamber housing portion and housed in a separate bearing housing portion axially spaced from said chamber portion to prevent cross leakage between said housing portions; replaceable sealing means on said shaft at least adjacent said high pressure end of said chamber portion and located between said chamber portion and said bearing housing portion; said sealing means comprising a radially split wear member interposed between ring shaped relatively rotating seal members.

8. In a screw compressor as specified in claim 7 the improvement as specified in claim 6 further comprising coating means on at least one of said rotors, said coating being non-corrodible by water and permanently displaceable when interference occurs.

9. The screw compressor improvement as specified in claim 7 wherein all of said bearing means are axially spaced from respective ends of said chamber portion and housed in separate bearing housing portions axially spaced from said ends of said chamber housing portion; and said sealing means on said shaft are outwardly adjacent both ends of said chamber housing portion.

References Cited UNITED STATES PATENTS 1,021,180 3/1912 Clifton.

1,319,776 10/1919 Kerr 230-143 2,491,677 12/1949 McCulloch 230-141 2,880,676 4/1959 Succop.

3,073,513 l/1963 Bailey 230-143 3,073,514 1/1963 Bailey et a1 230-143 3,265,293 8/1966 Schibbye 230-143 3,282,495 11/1966 Walls 23 0-143 3,414,189 12/1968 Persson 230-143 1,409,868 3/ 1922 Kien 230-205 1,597,411 8/ 1926 Kinney.

1,672,571 6/1928 Leonard 230-205 1,673,262 6/1928 Meston et al 230-141 2,475,550 7/1949 Larsen 277-87 WILLIAM L. FREEH, Primary Examiner W. J. GOODLIN, Assistant Examiner US. Cl. X.R. 418-85, 201 

